This invention relates to a discharge lamp operating system wherein the discharge lamp means is ignited by a high voltage in every half cycle of an a.c. power source in its continued operation. More particularly the invention relates to a combination of a single choke ballast and an electronic preheating circuit forming part of the "every half cycle" ignited system.
In recent years, energy optimization and the saving of energy resources have been the prime targets of technology. The present so-called "every half cycle" ignited discharge lamp operating system intends to find a solution for the saving of energy in the field of illumination. For example, in an article "An Investigation of Minimized Ballast upon Every-half-cycle Ignited Discharge Lamp Operating System with Analogue Computer Simulation" published in the Journal of Light and Visual Environment (Vol 1, No. 1, 1977) by the Illuminating Engineering Institute of Japan, there are disclosed theoretically estimated improvements in the power loss and the weight reduction. The power loss is reduced to one third and even up to one forth, as compared to the rapid start type. The weight reduction is to one fourth and even to only one eleventh compared to those of conventional switch-start or rapid-start discharge lamp operating devices. Thus, a substantial miniaturization of such device has been realized according to said article. To facilitate the understanding of how the current limiting choke, which is used in the "every half cycle ignited discharge lamp operating system" of the present invention, is miniaturized, the function of the conventional discharge lamp operating system be explained first. As shown in U.S. Pat. Nos. 3,665,243; 3,753,073; 3,866,088; and 3,942,069, a conventional fluorescent lamp operating device is constructed, for instance, of a circuit shown in FIG. 1, wherein the discharge lamp FL is connected in series with the current limiting choke CH, which acts as a current limiting device, and to an a.c. power source AC and the oscillation circuit or oscillator R1 is connected to the lamp FL in parallel. When the power source AC is turned on, the oscillator R1 starts oscillation and hence heating the filaments fa and fc of the lamp FL by its oscillation current. The oscillation output voltage which is higher than the required starting voltage Est, is applied across the filaments of the lamp. When the filaments fa and fc are sufficiently heated and the required starting voltage for the lamp FL becomes lower than Est, the lamp is ignited by the above-described oscillation output and begins a lag-phase operation. Once the lamp is lit, the terminal voltage vT, i.e. the lamp voltage of the discharge lamp FL corresponds to about one half of the source voltage. Hence, the oscillator R1 can no longer continue to oscillate, and the discharge lamp FL is operated by the voltage supplied from the power source AC through the current limiting choke CH.
FIG. 2 (A), (B), and (C) show the waveforms of the source voltage "e", the lamp voltage vT and the lamp current iT respectively obtained from observation of the lamp under operation. FIG. 2 (D) shows the product of the terminal voltage vCH of the current limiting choke CH which does not contain a resistive component, and the lamp current iT. FIG. 2 (E) shows the energy Se stored in the choke at each respective instant. As seen from these waveforms, for the period (t1-t2) in which the source voltage e is higher than the lamp voltage vT, the energy Sel= .sub.t1.sup.t2 (e-vT)iTdt increases monotonically and is stored in the current limiting choke CH. For the period (t2-t3) in which the source voltage e is lower than the lamp voltage vT, the stored energy is released and the released energy is expressed by Se2= .sub.t2.sup.t3 (e-vT)iTdt. The size of the current limiting choke CH is determined by the maximum value of the stored energy Se shown in FIG. 2 (E). Thus, the capacity of the choke CH must be so selected as to endure the maximum amplitude Semax of the stored energy.
In the just described case, the required reignition voltage Erst of the discharge lamp FL must be lower than the source voltage e at the reignition instant. In other words, the peak value vTP of the lamp voltage vT must not exceed the source voltage e. Since in the conventional discharge lamp the effective value VT of the lamp voltage is set at about 1/2 of that of the source voltage e, the effective value VCH of the terminal voltage of the current limiting choke is set higher than the 1/2 of the source voltage.
My U.S. Pat. 4,079,292 granted on Mar. 14, 1978, discloses an "every half cycle ignited discharge lamp operating system", and present FIG. 3 shows one example of such operating circuits for a fluorescent lamp. In FIG. 3 the current limiting choke CH and the discharge lamp FL, connected in a series circuit, are connected to an a.c. power source AC. The current limiting choke is provided with a secondary winding W20 which superposes the oscillation output of the booster circuit as a high voltage generating means, on the power source voltage, and one end of the secondary winding W20 is connected to a junction j1 of the filament fa of the discharge lamp FL. The other end of the winding W20 is connected to the booster circuit or booster R.
The booster R is a series connection of an intermittently oscillating capacitor C1 and an oscillator R1 which is connected in a parallel circuit of an oscillation capacitor C and a series connection of a bidirectional diode thyristor S and a backswing voltage generating or boosting inductor L. One end of the booster circuit R is connected to the other end of the secondary winding W20. The other end of the booster R is connected to a junction j2 of the filament fc of the discharge lamp FL. In addition, the booster R may be, so far as it operates as a high frequency oscillator, replaced by a circuit which employs a gated type thyristor such as a TRIAC or by an inverter or a high voltage generating circuit which employs a pulse generator.
The above circuit operates as follows. When the power source AC is turned on, the source voltage e is applied to the discharge lamp FL through the current limiting choke CH and to the booster R through the secondary winding W20. In the booster R the source voltage e is applied to the thyristor S through the intermittently oscillative capacitor C1, then the oscillator R1 starts oscillation in order to break over the thyristor. Without the intermittently oscillating capacitor C1 the oscillation would continue once generated, but in the circuit shown here due to the capacitor C1, the oscillation occurs repeatedly at every half cycle in the leading portion of the source voltage e.
After the oscillator R1 starts oscillation, the capacitor C1 is charged with such polarity that it cancels the source voltage e. Thus, as the terminal voltage VC1 rises and the voltage difference of the terminal and the source voltage e becomes insufficient relative to the break-over voltage VBO of the thyristor S, the latter assumes the off-state, hence the oscillator R1 ceases to oscillate. Therefore, the terminal voltage vc1 of the capacitor C1 remains constant and the oscillator R1 is in the non-oscillating state during the later period of the just described half cycle. In the next half cycle, where the source voltage e is reversed compared to the preceding half cycle, the summed up voltage of the source and the terminal voltage vc1 which charged the capacitor C1 during the preceding half cycle, is impressed on the oscillator R1, which causes the break-over of the thyristor and hence again an oscillation. However, from the moment when the oscillation starts again, the intermittently oscillating capacitor C1 begins to be charged to compensate the source voltage e again and the polarity of the terminal voltage vc1 of the capacitor C1 is rapidly reversed. In the meantime the oscillator R1 again stops oscillating. The oscillator R1, therefore, operates exclusively during the period of the inversion of the voltage of the intermittently oscillating capacitor C1. Thus, the current flows from the source AC to the oscillator R1 through the primary W10 and the secondary winding W20 of the ballast choke CH solely during this period. Such operational mode is repeated in the same way in every following half cycle. FIG. 4 (A) is a voltagecurrent waveform showing the above described operation, wherein e is the source voltage and vc1 is the terminal voltage of the intermittently oscillating capacitor C1. As seen from this figure, the current ic1 flows to the capacitor C1 in the period of the rapid inversion of the terminal voltage, and just in this period also appears the oscillation output vR of the high frequency high voltage across both ends of the booster R.
The above described oscillation output vR is blocked by the primary W10 and the secondary winding W20 of the current limiting choke CH. The terminal voltage of the primary winding W10 is superposed on the source voltage e, and the summed up voltage is impressed on the discharge lamp FL and the filament preheating circuit PRH. Thus, in the filament preheating circuit PRH the voltage is impressed on the thyristor SP through the high frequency blocking inductor NL, then the thyristor SP is driven to conduct by the sudden change effect (dV/dt effect) of the voltage. Therefore, at the end of the intermittently oscillative phase, the current flows from the source AC through the filament fa, the thyristor SP, the inductor NL and the filament fc, so as to initiate the preheating of the filaments fa and fc. Each time when the oscillation output vR of the booster R is applied to the preheating circuit PRH, the thyristor SP is driven to conduct and the preheating is repeated during this period by the current flow from the source AC to the filaments fa and fc.
When the filaments fa and fc are sufficiently preheated and the voltage required to start the discharge lamp FL becomes the starting voltage Est, the discharge lamp FL is triggered to start by the oscillation output vR from the booster circuit R. After the discharge lamp FL is lit, the intermittent oscillation energy flows mainly through the conducting discharge lamp FL and the remaining energy is absorbed by the high frequency blocking inductor NL. By setting the break-over voltage VBO of the thyristor SP sufficiently high above the peak voltage vTP of the lamp voltage, the thyristor does not conduct. In addition, in the case where the breakover voltage of the thyristor is set considerably high, if required, the high frequency blocking inductor NL can be omitted. After the lamp is lit, the filament preheating does not occur, hence, the discharge lamp FL is ignited by the oscillation output vR at every half cycle of the power source AC and operated solely by the source voltage e as referred to in FIG. 4 (B). In addition, the preheating circuit PRH of FIG. 3 may be replaced by a filament preheating transformer.
FIG. 5 shows the waveforms of each instant observed in the experiment using the circuit of FIG. 3, wherein the high frequency component is neglected. As seen from FIG. 5 (B), the lamp voltage vT is a nearly rectangular waveform having a pause period due to the intermittently oscillating period. Thus, the effective value VT of the lamp voltage is about 90-95% of the conventional lighting system. The discharge lamp FL is reignited at the step-up portion of every half cycle by the high voltage oscillation output vR in the following way. As the intermittent current ic1 originating from the booster circuit flows through the secondary winding W20, the corresponding terminal voltage of the winding is applied through the coupling with the primary winding W10, as a rapidly rising low frequency voltage, to the discharge lamp FL, whereby a sufficient quantity of ions are generated in the lamp, which is helpful for its easy breakdown, thus, resulting in the arc discharge of the lamp. Further in this case, the lamp current iT remains constant in the step-up, in spite of the change of the source voltage e, and as the lamp current iT increases, the above current ic1 decreases due to the encroachment of the edge of the lamp current waveform into the next half cycle. Hence, the initial value of the lamp current is controlled to be rather low by the rapidly growing low frequency voltage. Consequently, the fluctuation of the lamp current of the every half cycle ignited operating system is excellent regardless of the decrease in the stabilizing impedance.
The lamp current from the power source AC to the discharge lamp FL flows, for the most part, during the period (t2-t4) outside of the oscillation period, as shown in FIG. 5 (C). During the periods (t1-t2) and (t4-t5), the current ic1 flows from the source AC to the booster R. The waveform ic1 is shown in FIG. 5 (D). This current flows both through the primary winding W10 and the secondary winding W20, which are coupled together by the current limiting choke CH in a magnetism increasing manner or direction. The exciting or energizing effect can generally be varied by the winding ratio of the primary W10 to the secondary winding W20. FIGS. 5 (E) and (F) show the waveforms of the stored energy Se and the product (vCH.i) of the voltage of the current limiting choke CH calculated from the above described waveforms of the lamp volage vT, the lamp current iT, the current ic1 which flows into the booster R and the source voltage e. FIG. 5 (E) shows the voltage-current product of the current limiting choke CH caused by the voltage difference between the source voltage e and the oscillation output vR or the lamp voltage vT. The total stored energy Se1 due to the current ic1 is given by {Se1= .sub.t1.sup.t2 (e-vR)Kicldt}, wherein K is a constant determined from the winding ratio of the primary W10 and the secondary winding W20. The stored energy Se2 for the period, where the source voltage e is higher than the lamp voltage vT, is given be {Se2= .sub.t2.sup.t3 (e-vT)iTdt}.
On the other hand, during the period (t3-t4), where the lamp voltage vT is higher than the source voltage e, the stored energy is released and the total released energy Se3 is given by {Se3= .sub.t3.sup.t4 (e-vT)iTdt}. Consequently, the energy, which is stored in the current limiting choke CH, is varied as shown in FIG. 5 (F). The relation Se1+Se2=Se3 applies to the waveforms shown in FIG. 5.
The calculations of the stored energy in the ballast of the conventional operating system and in the every half cycle operating system, are based on the waveforms shown in FIG. 2 and FIG. 5 respectively. The following relation is obtained, for instance, for the cases when a lamp of the 40T12 type is operated by the former conventional system with a 200 v linevoltage and the same lamp is operated by the latter in an "every half cycle system" with a 100 v line-voltage. The results may be compared as follows: ##EQU1## Thus, the current limiting choke CH of the latter can be reduced in impedance and in size accordingly.
Substantial advantages may thus be obtained from an "every half cycle ignited operating system" which is the aim of the invention so far described. However, still some problems remain to be solved. Namely, presently there are few discharge lamp types which have the lamp voltage equal to the source voltage. Therefore, an appropriate voltage difference exists in the conventionl type discharge lamps. In another aspect, since drawing out of the intermediate tap of the choke coil CH is necessary, such a structure imposes an unfavorable condition on its fully automated production.
Another problem is seen in that the preheating circuit means are rather complicated which also causes an economical disadvantage when one or more discharge lamps with filaments are to be operated. For instance, where a plurality of discharge lamps are to be employed in series, a preheating means which meets the requirements of small size, light weight and low cost cannot be realized satisfactorily with conventional preheating means employing the secondary windings of a heating transformer connected on the power source side of the discharge lamp means.
It has also been found to be somewhat difficult to maintain a stable operation in every half cycle due to the variation of the source voltage and/or the ambient temperature. The source voltage usually varies .+-.10% at its maximum. Further, the lamp voltage of the discharge lamp means varies according to the variation of the lamp current and ambient temperatures. For instance, as the ambient temperature falls or rises the lamp voltage goes down and it may happen that if the source voltage e goes up it reaches a value more than twice as high as the lamp voltage vT. On this occasion the discharge lamp may operate not according to the "every half cycle operational mode" but according to a conventional operational mode as illustrated in FIG. 1 and the discharge lamp operational mode shifts back ans forth between these two modes, resulting in a flickering light output and a greater stress on the current limiting means.